1. Field of the Invention
The present invention relates to cutting tools and, in particular, to coated reinforced ceramic cutting tools.
2. Description of the Prior Art
In a myriad of applications throughout modern industry, it is necessary to form objects which resist deformation into a final desired shape. One common means of shaping such objects, known as workpieces, is machining. Various known machining processes include turning, shaping, planing, milling, facing, grinding, sawing, boring and drilling. In order for the workpiece to be shaped by a machining process, relative movement must occur between the workpiece and the instrument, known as a cutting tool, employed to shape it in order that a desired amount of material may be removed from the workpiece.
To meet the productivity demands currently imposed on the machining industry, the rate of removal of material from the workpiece is of critical concern. The commonly measured parameters of cutting tool performance include predictability of tool performance failure, cutting speed, depth of cut, feed rate and tool life. To meet this end, those skilled in the art have previously developed materials for use as cutting tools which are intended to allow greater rates of productivity in machining operations. Such previous materials may be broadly classified as either xe2x80x9cmetalsxe2x80x9d or xe2x80x9cceramicsxe2x80x9d based on the constituents of the materials. The term xe2x80x9cmetalsxe2x80x9d generally includes tool steels, high speed steels, cast nonferrous alloys and sintered carbides.
Those skilled in the art recognize that cutting tools made from such metals possess the desirable characteristic of a high degree of toughness. Toughness is known as the ability of a material to resist crack propagation and, hence, to be less susceptible to breakage. However, a notorious flaw of metal cutting tools is their characteristic of rapid wear under service conditions. The wearing of metal cutting tools arises both from the mechanical wearing away of the cutting tool due to the cutting activity and from the chemical breakdown of the tool at the high temperatures created by the frictional forces generated during high speed cutting. The chemical reactivity of metal cutting tools can also chemically degrade the workpiece causing a rough surface finish which must be subjected to costly secondary finishing procedures. To lessen the effects of the problems associated with metallic cutting tools, the common practice in the industry has been to drastically lower the cutting speeds at which metal cutting tools are employed. The lowering of the cutting speeds necessarily causes the lessening of the productivity rate of the machine using the cutting tool and forces the machining entity to employ additional machines and personnel to achieve a desired level of productivity.
The other category of cutting tools referenced above is that composed of ceramic materials. Ceramic cutting tool materials include aluminum oxide, or alumina, and nitrides such as silicon nitride. Due to their chemical composition, ceramic cutting tools are much more resistant to heat than their metallic counterparts. As such, the cutting speeds at which ceramic cutting tools may be used are increased. Also, ceramics are generally nonreactive with the metal workpiece at elevated temperatures thereby reducing concerns relative to the chemical degradation of the workpiece which also increases the cutting speeds at which ceramic cutting tools may be used.
Ceramic cutting tools, however, are fraught with a serious shortcoming of their own. It is known that ceramic cutting tools do not possess strengths similar to metals and fail catastrophically and unpredictably. Such catastrophic, unpredictable failures have led those in the industry to only employ a particular ceramic cutting tool for a most limited number of workpieces. In fact, in many instances ceramic cutting tools are only used once and then removed from the cutting machine and discarded. It will be readily appreciated that the constant halting of the manufacturing process to allow each ceramic cutting tool to be removed from the machine and replaced with another is very costly in terms of machine downtime and cutting tool expenses and, additionally, is extremely labor intensive. Many manufacturing installations have been forced to purchase, operate and maintain a multiple number of machines to accommodate the excessive amount of downtime experienced by a single machine using ceramic cutting tools which must be constantly replaced. Moreover, due to the unpredictable failures of ceramic cutting tools, they are not suitable for use on automatic machines which are useful in efficiently producing a large number of workpieces.
In an effort to develop materials for use in cutting tools which avoid the problems associated with metallic and ceramic tools, various solutions have been proposed by the prior art. One such solution which has been employed with respect to metallic cutting tools has been to coat such tools with a layer of a harder, more wear-resistant material such as titanium carbide. Cutting tools so coated have been shown to provide an improved tool life as the coating provides a barrier against the mechanical, thermal and chemical effects of cutting on a metallic cutting tool alone. However, due to the basic properties of metallic cutting tools, even metallic cutting tools so coated have proven to have tool lives less than those of ceramic cutting tools under high speed cutting conditions.
It has additionally been proposed to coat ceramic cutting tools such as those formed of silicon nitride with a material such as alumina. The coating of a ceramic cutting tool provides a more wear-resistant cutting tool as the coating acts as a thermal barrier to resist any grain boundary degradation which may occur in ceramic cutting tools at elevated temperatures. In addition, certain ceramic cutting tools have been found to be chemically reactive when cutting materials such as steels at high speeds. The coating of alumina, therefore, decreases the chemical effects on the ceramic cutting tool. It should be recognized that the coating of a ceramic cutting tool with a coating layer has proven to have little or no effect on the ability of the tool to resist catastrophic failure. As such, and due to the increased tool cost due to the coating process, coated ceramic cutting tools have not, heretofore, achieved a significant degree of commercial success.
In another attempt to eliminate problems with prior cutting tool materials, there has recently been developed a material which minimizes the potential for catastrophic failure in a ceramic cutting tool. The specifics of such material are disclosed in U.S. Pat. No. 4,543,345. That material consists of a ceramic matrix having distributed therethrough a reinforcing material comprising ceramic whiskers. In a preferred embodiment of that material, the ceramic matrix comprises alumina. The most preferred of the reinforcing ceramic whiskers are those of silicon carbide and are included in the cutting tool in an amount equal to or less than 50% by volume of the material.
It has been found that a cutting tool material as described immediately above provides an increased tool life over a purely ceramic cutting tool as the toughness and, hence, resistance to breakage is increased. As such, the resistance to catastrophic unpredictable failure is increased. However, it has also been discovered that the cutting tool so produced contains a previously unseen shortcoming in ceramic cutting tools. Due to the inclusion of the silicon carbide whiskers throughout the ceramic matrix, a certain amount of the ceramic whiskers are necessarily disposed adjacent to the surface of the cutting tool. As the cutting speed and, hence, the heat due to friction, is increased, although not to the level harmful to previous ceramic cutting tools, a chemical reaction has been found to occur between the surface silicon carbide whiskers and the metal workpiece. The problems caused by such a chemical reaction parallel those faced by purely metallic cutting tools such as decreased tool life and chemical damage to the workpiece. Those problems are found to occur at a higher temperature than that of a metallic cutting tool but at a lower temperature than a ceramic cutting tool. In addition, when the silicon carbide in the whiskers is exposed to the metal cutting interface temperatures, it will oxidize to silica and thereby provide a path for oxygen to enter the substrate and weaken the structure.
While previous attempts to cure certain of the shortcomings of prior art cutting tools have included the deposition of a coating layer thereon, such a procedure is contraindicated relative to ceramic cutting tools such as those which are alumina based. Applicant has discovered that the deposition of a coating layer of alumina on a ceramic cutting tool formed of alumina produces a decreased tool life as compared to an uncoated cutting tool of the same substrate material. In fact, the decrease in tool life of the coated alumina composition is so marked as to clearly indicate that such a coating is extremely detrimental to the tool life of an alumina cutting tool.
The subject invention is directed toward an improved reinforced ceramic cutting tool which overcomes, among others, the above-discussed problems and which provides a cutting tool capable of use at high speeds without experiencing catastrophic failure or excessive tool wear.
In accordance with the present invention, there is provided an improved material for use in applications such as for a cutting tool in machining operations. The material provided herein is formed from a substrate of a ceramic matrix having distributed therethrough a reinforcement comprising ceramic whiskers, which substrate is coated with a layer of a substance such as alumina.
According to a preferred embodiment of the invention, the ceramic matrix is formed of alumina, either pure or with inclusions of modifying materials. Alternatively, the matrix may be formed of silicon carbide, silicon nitride, titanium diboride, titanium carbide, titanium carbonitride, titanium nitride, zirconium oxide, zirconium carbide, boron carbide, boron nitride, hafnium carbide, hafnium nitride, tantalum carbide, niobium carbide, tungsten carbide, aluminum oxinitride and SIALONS (a known combination of silicon oxide, oxygen, nitrogen and alumina). The preferred material for the ceramic reinforcing whiskers is silicon carbide. Other materials suitable for the ceramic whiskers include silicon nitride, alumina, titanium diboride, titanium carbide, titanium carbonitride, titanium nitride, zirconium oxide, zirconium carbide, boron carbide, boron nitride, hafnium carbide, hafnium nitride, tantalum carbide, niobium carbide, tungsten carbide, aluminum oxinitride and SIALONS or any other material which is compatible with the ceramic matrix and with which it forms adequate reinforcement bonding.
The level of whisker content in the ceramic matrix should not be so great as to degrade the properties of the matrix. However, if the whisker content is too low, sufficient reinforcement will not be provided. As such, the generally acceptable range of whisker content will be about 2-40% volume percent whiskers in the composite materials.
The coating material applied to the substrate is preferably selected from the group consisting of silicon carbide, silicon nitride, alumina, titanium diboride, titanium carbide, titanium carbonitride, titanium nitride, zirconium oxide, zirconium carbide, boron carbide, boron nitride, hafnium carbide, hafnium nitride, tantalum carbide, niobium carbide, tungsten carbide, aluminum oxinitride and SIALONS or a combination thereof, but most preferably is alumina and is applied by means of a chemical vapor phase deposition (CVD) process or by physical vapor deposition (PVD), Sol-gel or ion implantation processes or a combination thereof. The alumina coating is preferably applied to a thickness of approximately 5 microns.
Applicant has discovered that the application of an alumina coating to the reinforced ceramic cutting tool markedly increases the tool life while providing a tool which resists unexpected catastrophic failure. As discovered previously, the application of an alumina coating to an alumina-based cutting tool is known to degrade the tool life. As such, Applicant""s invention provides unexpected results which not only show an improvement in alumina-based cutting tool life because of the addition of an alumina coating thereto but which, in fact, are taught away from by the prior art.
Accordingly, the present invention provides solutions to the aforementioned problems present in the machining industry. As this invention provides an improved cutting tool which has the benefits of ceramic cutting tools without the shortcomings thereof, machining processes may be carried on rapidly, efficiently and without the requirement of a great number of machines to accommodate a significant productivity rate.
These and other details, objects and advantages of this invention will become apparent as the following description of the present preferred embodiment thereof proceeds.